Introduction

A Bandonion or Bandoneon
is some sort of square headed accordion,
as I read somewhere.

Well, this is a quite unprecise explanation: Accordion and
bandoneon are distinct free-reed bellow instruments. But there
are so many variants of instruments based on this principle out there,
that even specialists are sometimes confused.
It is normal that you don't know exactly what it is.
The Bandoneon, originally Bandonion, has itself many variants. I'll try
a definition:

A Bandoneon is a German Concertina with a particular keyboard layout.

This statement may cause controversies, because its keyboard experienced
many changes in respect to where to place a certain button or note.
In contrast to usual keyboards with a systematic order, like a piano,
the bandoneon uses buttons placed in several rows and which must be
reached by a more or less fixed hand position.

Now, what is a German Concertina?

A German Concertina is a handheld free-reed
instrument (aerophone) with a bellow of a nearly square cross
section with buttons on both sides for individual notes as to perform
arbitrary harmonies.

In contrast, the Wheatstone Concertina, an early
English Concertina, has a hexagonal bellow cross-section,
also with buttons for single notes, but which are distributed to both hands,
thus disregarding harmonies. On the other side, the scales include
2 en-harmonics, which declares its use as melody instrument.

An accordion, however, performs complete predefined chords by depressing only
one button or key.

The wrist centered position used for bandoneons and concertinas,
allow to place only a limited number of buttons.
To cover a certain extension, most buttons underlay two notes,
one on push and a different on draw. The assignment of the notes was governed
by the need of easy access to the notes building the harmony.
The note distribution
of the resulting keyboard appears to be chaotic and takes some time to memorize.
The early double action instruments were all tuned diatonic, which led
to confuse the term.
This complicated double action principle is not necessary for the
accordion with its single action keys.

While a linear keyboard offers obviously advantages regarding chromatic
scales, the one for bandoneon needs a well argument placement of
the single notes in order the facilitate the composition of harmonies,
particularly for the left hand, usually destined to the
accompaniment, and the melody favored arrangement of the right hand.

The free reed type is the last discovered mechanical sound producing
principle and is applied in very many different music instruments as
it offered a wide field of experiments and innovations.
Manufacturers invented new instruments for specific genres
hoping to match their needs.
Musicians were requesting melody instruments, chromatic ones for all
tonalities, for soloists, ensembles or orchestral integration.
Others preferred
single action buttons to simplify the performance.
One way to strengthen
the sound is to install two or more reeds for each note by adding
registers, in a fixed
or switchable manner. Soloists needed an extension of the tonal range.

In all cases, weather possible or not, an enlargement of the box was needed,
thus reducing the portability. In consequence, very many instrument
versions existed. Some of the measures do modify the sound.
The most significant influence on the sound as achieved by the number
of reeds per note. Having more than one reed per note,
there are several ways to tune the instrument which has a great
impact on the sound. But each reed added to a note, increases the size
and the weight ans also the air consumption.
The total volume of the instrument, also, though less dramatic,
changes the sound. If we regard the period from 1854 to 1930,
we may point to 3 main bandoneon variants:

Strictly speaking, a bandoneon should follow the design of Heinrich Band
of Krefeld / Rhineland, hence Rheinische Lage. All other variants fall back
to the class Concertina. Under this severe classifying, only the first
in the list may be considered a bandoneon, allowing also those
with additional registers. If we are more flexible, we may regard
the way notes were added to a particular keyboard in order to extend
the tonal range: in case of the bandoneon the additions were made at the
peripheral regions of the original core in a circular fashion, while
for the concertinas the linear concept is followed.
This new criterion may be used for a further distinction of model types.

One important property of the free reed principle is the extraordinary
precision of the tonal pitch. Errors of intervals are clearly audible.
Most early free reed instruments were tuned diatonic.
or intended deviations in tuning are immediately audible.
This phenomenon becomes a problem, and buttons for the en-harmonics
♭e and ♭a were provided.
During the middle of 19th century smaller harmonicas were
tuned diatonic. Few early diatonic boxes were tuned even mid-tone.

The bandoneon, for at least since the late 19th century,
was tuned tempered with 12 halftones per octave.
The consequence was to become used to the produced sound.

What can be done?

The earliest versions of free reed instruments had one reed per note.
The produced sound is sweet and dull (see and hear).
Possibly in 1856, but cetrtainly in the late 1860's instruments
with more than one reed per note appeared.
The additional reed permits sound variants:

A dual reed arrangement may be combined as follows:

the fundamental 8' with a lower octave 16'

the fundamental 8' with upper octave 4'

the fundamental 8' double or third with tremolo

(two identical reeds don't modify the sound)

While the octave tuning results in a stronger and sharper sound,
the tremolo results in a sweeter character. Since then
the accordion applied the tremolo trick to hide the problem raising
from the even temperament.

The bandoneons used in Bavaria, are very similar to those used in tango,
even the button layout is very similar to the original rheinische Lage,
but are tuned with tremolo. The resulting sound iss close to an accordion.
Some other constructive details are characteristic for
the Schrammel Bandoneon.

The characteristic bandoneon sound in tango comes mainly
from the dry tunig wich consists of tuning the dual reeds
exactly one octave apart with absolutely no tremolo.

This type is the symbol of tango,
a popular music originated at the borders of the
Rio de la Plata. It was the tango that saved us this instrument from
its extermination and its revival makes us aware
of the instrument's existence. That's also why the instrument's name
changed from its original Bandonion, given by
Heinrich Band's Akkordion, to the Spanish Bandoneón.
Heinrich Band, a music teacher and dealer, was the prominent promoter of this
Instrument, though he did not invent the instrument. His merit is to
have modified and extended the until then used Chemnitzer.

After WW II the East German Language Council accepted the term
Bandoneon as a regular denomination besides the original one
honoring the largest market being still the River Plate area.

If you are looking for a tango bandoneon, you may find it labeled
142 voice . However there are some of them with 152 voices.
One button has 2 voices, so the 142 voice instrument has 71 buttons.
The models used in Germany are of 144 voices.

The high popularity of the bandoneon during the 20ies, forced the
French musette players to learn this instrument too. However,
it was too complicated and time consuming to learn the bandoneon
while they were familiar to their accordions. There was a demand
for a substitute with single action buttons but of identical appearance
on an rheinische Lage bandoneon.
The unisonoric so-called Peguri
Chromatic Bandoneon
designed by Charles Péguri in 1925/6 is based mechanically on a
142 voice bisonoric bandoneon. They were built by the same manufacturer
of the traditional ones. At that time it was used to call double
action'instruments as diatonic and single action were chromatic.
This old nomenclature may lead to confusion upon the term diatonic.
The first instruments were double action and diatonic, which means
they were designed for certain tones only. The truth is that both cover all
12 tones. Today double action means bisonoric and single action unisonoric.

While the south American market was fixed on the once introduced model,
in Germany they continued to experiment and modify (develop) upon this
instrument type. The result was an unnumbered different keyboards versions
making them too expensive. They decided to unify the design and make only one
for concertinas and bandoneons. As already said,the concertina is more
linear while the bandoneon follows a circular way of not position. Finally
a Einheitsbandonion and one Einheitskonzertina was created.
This was a great step to a standard way for fingering written music and
the end of strange constructions. Different models with 2 and 1/2 registers,
3 and 4, in special cases up to 5 registers were possible and there was no
problem to switch to an other model. However, the result was a fairly large
box and the advantage in respect to other free reed instruments was no
longer evident. Also the sound was close to that of an accordion.

But developers did not resign. There exist many other layouts, mainly
single action.

Most people think the bandoneon is related exclusively to
tango. It is true, that the current tango wave did remember us the
bandoneon, saving it from the final extermination. However, the introduction
of the bandoneon into tango occurred around 1900. before that time, the main
repertory was dance music: among the 520 pieces published by Johann Klein in
Vienna 1875, we find 350 polkas, mazurkas, waltzes and quadrilles.
(see: Dunkel, Akkordion, Bandonion, Concertina)
A similar
repertory was published in Germany and it is proofed that it was "fresh"
material. Only one year after the famous tango El Choclo by
Angel Villoldo was published for pianoforte by 1913, a version for bandonion
was available. The second most popular genre for bandoneon were the marches.
It is true that there existed also arrangements and excerpts of operas, arias,
other dances, chorals, hymns, and melodies of other countries for the
bandoneon. No evidence was found for the supposed significant use in churches.

The tonal range is that of a harpsichord. Original music for this instrument
is particularly handy to be transcribed for bandoneon.